It is expected that particles of metal oxides or metal hydroxides which are sharp in particle size distribution are favorably usable for a variety of applications, for example, as raw materials for ceramics, fillers for resins, spacers for liquid crystal display devices and the like. Expected is the use as insulators for liquid crystal spacers of particles of metal oxides or metal hydroxides, particularly those which have a relatively large particle diameter such as greater than 0.1 .mu.m and which are sharp in particle size distribution thereof.
That is, liquid crystal display devices are widely used as display devices for watch, computer or wall-hanging television. This liquid crystal display device is a display device in which liquid crystal is used, the molecular arrangement of which changes merely by application of a slight voltage resulting in variations in the direction of deflection. Such a display device usually has a structure comprising two plate electrodes having interposed a liquid crystal layer therebetween.
In the liquid crystal display devices of the kind, it is desired that a thickness of the liquid crystal layer is thin and uniform as far as possible. If the thickness of the liquid crystal layer is not uniform, partial unevenness is brought about in the strength of electric field to be applied to the liquid crystal layer, with the result that a contrast ratio of the image varies depending on the local area of the liquid crystal layer, resulting irregularities of the image. Furthermore, a rate of response to an input signal of liquid crystal will vary according to the thickness of the liquid crystal layer and to the strength of electric field. If, therefore, the thickness of the liquid crystal layer is not uniform, a difference will develop in the rate of response, and no sharp images come to be obtained.
For the reasons mentioned above, it has been a usual practice in the manufacture of liquid crystal display devices that a spacer consisting of a thin insulator is interposed between two plate electrodes and liquid crystal is filled therebetween to form a thin and uniform liquid crystal layer between the two plate electrodes.
As spacers adopted in the liquid crystal display devices mentioned above, there have been used spherical aluminum oxide for abrasives which has been classified into particles of 2-10 .mu.m, glass fiber of 2-10 .mu.m in diameter which has been cut into pieces of 50-100 .mu.m in length, or a synthetic resin such as benzoguanamine which has been formed into spheres of 2-10 .mu.m.
In the liquid crystal display devices employing such conventionally known spacers, no particularly serious problem is brought about when the devices are of small size, but such problems as mentioned below are encountered in the devices when they are of large size.
(a) It is necessary to vary delicately a thickness of liquid crystal layer according to the kind of liquid crystal used, but it is practically impossible to control the shape of spacer capable of coping with this delicate variation in thickness of the liquid crystal layer.
(b) It is not possible to provide a liquid crystal layer having a uniform thickness because the spacer particles are broad in particle size distribution, with the result that irregularities occur in the displayed images or abnormal color tone is brought about in certain cases.
(c) Where a strong dielectric liquid crystal is used. It is necessary that the liquid crystal layer be made to have a thickness of about 1-2 .mu.m. However, no such spacer capable of controlling a thickness of liquid crystal layer so as to have the thickness mentioned above.
(d) There are some cases wherein spacer particles in the liquid crystal layer aggregate and come to be visible to the naked eye in the displayed images or spacers having as long axis of 10-50 .mu.m come to be visible themselves to the naked eye in the displayed images.
(e) When spacer particles used are not spherical, the spacer comprising the same sometimes damages transparent electrodes, whereby the resulting liquid crystal device is found to be a defective unit.
(f) Where a resin is used as a spacer, it apts to undergo change in shape when heat or pressure is applied thereto, whereby no liquid crystal layer having a uniform thickness can be obtained.
Under these circumstances, desired is the advent of such spacer particles consisting of metal oxides or metal hydroxides for use in liquid crystal display devices as capable of solving the above-mentioned problems.
If black spacer particles consisting of metal oxides for use in liquid crystal devices are available, the display devices excellent particularly in point of contrast of the displayed images are obtained, and hence desired ardently is the advent of such black spacer particles mentioned above.
Furthermore, spacer particles consisting of metal oxides or metal hydroxides for use in liquid crystal display devices, which have been coated on the surface thereof with synthetic resins, do not move in the display devices, and hence the advent of such resin coated spacer particles is also desired.
In preparing particles of metal oxides or metal hydroxides mentioned above, for example, silica particles, there has heretofore been known a process wherein silicic acid obtained from water-glass is allowed to grow to obtain silica sol having dispersed particles of silica having an average particle diameter of 0.05-0.1 .mu.m in a liquid. In the case of the thus obtained silica sol, however, miscells are formed if the particles in the sol have a particle diameter of less than 0.05 .mu.m, and if the particles have a particle diameter exceeding 0.05 .mu.m, particle size distribution thereof becomes sharp but the particles mono-dispersed in the sol are few in number. Moreover, there were also involved in this process such problems that it is difficult to obtain particles having a particle diameter of greater than 0.1 .mu.m and it takes a long period of time such as from several days to scores of days while keeping the reaction system at elevated temperatures if the particles having a particle diameter of greater than 0.1 .mu.m are intended to obtain, and, at the same time, new seeds are apt to form in the reaction system during the particle growth stage and consequently a particle size distribution of the resulting particles becomes broad. Accordingly, it is not possible, according to this process, to obtain on a commercial scale the mono-dispersed particles having a particle diameter of greater than 0.1 .mu.m and a sharp particle size distribution.
There has also been known a process for obtaining metal oxide or metal hydroxide particles having a particle diameter of about 0.05-2 .mu.m by hydrolyzing metal alkoxides in the presence of alkali catalysts in a water-alcohol system solvent. This process has such advantages that there is no need for keeping the reaction system at elevated temperatures and particles having a particle diameter of about 1-2 .mu.m and a relatively sharp particle size distribution are obtained by the reaction for several hour to scores of hours. In the process, however, there were involved such problems that when particles to be obtained by this process are, for example, those of silica, titania or zirconia, the particles having such properties as mentioned above are obtained only when the particle concentration in the reaction system is less than 0.5% by weight in terms of an oxide concentration, and consequently the productivity decreases, large amounts of organic solvent such as alcohol are needed and the cost of production increases. In this process, moreover, when the particles aimed at are those of silica, the particle diameter of the particles obtained is 0.2-0.3 .mu.m at the most. If the particles of having a particle diameter of greater than 0.5 .mu.m are aimed at in this process, the starting metal alkoxide used in the process must be those having a slow rate of hydrolysis, i.e. alkoxides containing long ester groups having about 5-7 carbon atoms. However, such specific alkoxides are expensive, and the use of these alkoxides involves such problems that because of a slow rate of hydrolysis, a long period of time is required for carrying out the reaction, the alkoxides are liable to undergo such influences as the composition of the reaction system, reaction temperature, stirring method, stirring speed, etc., and mono-dispersed metal oxide or metal hydroxide particles of stable quality cannot be obtained.